The invention relates generally to a method of monitoring a source contact in a flash memory. More particularly, the present invention relates to a method of monitoring a source contact in a flash memory by which whether a source contact having a narrow contact area contacts or not can be easily monitored using over-erase cell characteristic in a flash cell, in a flash memory device in which a source line is formed by a local interconnection method.
In general, a source line in a flash memory may be formed by diffusion line method by which impurity ions are implanted between sources of each of cells to form a diffusion line so that the source line can be formed inside a semiconductor substrate, or may be formed by applying a local interconnection method. In case that the source line is formed by local interconnection method, there are advantages that not only its effective cell size is reduced to about 86% compared to that when the diffusion line method is applied, but also a back bias effect by which the potential difference generated due to diffusion resistance is reduced from 0.1 V to 0.01 V can be reduced to {fraction (1/10)}, since the source contact formed per sixteen (16) cells needs not to be formed. However, as the flash memory device having the source line formed by the local interconnection method has a narrow source contact area in each of cells, there is a possibility that it may cause contact defects.
FIG. 1A is a plan view of a cell array in a conventional flash memory to which the conventional local interconnection method is applied, FIG. 1B is a cross-sectional view of a flash memory taken along lines X-Xxe2x80x2 in FIG. 1A and FIG. 1C is a cross-sectional view of a flash memory taken along lines Y-Yxe2x80x2 in FIG. 1A.
In the flash memory to which the conventional local interconnection method is applied, a plurality of unit cells consisted of a floating gate 13, a control gate 14, a drain 15 and a common source 16 is formed in a semiconductor substrate 11 in which an active region is defined by forming a field oxide film 12, control gates 14 in each of the unit cells are connected to form word lines, drain contacts 17 are formed in the drains 15 at each of the unit cells, respective drain contacts 17 are connected to form a bit line 170, source contacts 18 are formed at the common sources 16 in each of the unit cells and respective source contacts 18 are connected to form a common source interconnection line 180.
As can be seen from the plan view of FIG. 1A, 1024 source contacts 18 are in parallel connected. Thus, the conventional local interconnection method can increase a memory characteristic since it has a lower resistance value than that by the diffusion line method. As can be seen from FIG. 1C as viewed from the bit line 170, a contact defect may be generated since the contact area is narrow. Therefore, the contact state of the source contact 18 must be carefully monitored. This, however, could not be performed exactly with the conventional method. As can be seen from FIG. 1B, the convention method could not verify any existing defected contact of 1024 contacts, if exists. This is because, even though any voltage is applied to one end of the common source interconnection line, current will flow regardless of the defected contact since the common source interconnection line is connected.
It is therefore an object of the present invention to provide a method of monitoring a source contact in a flash memory by which whether a source contact having a narrow contact area contacts or not can be easily monitored using over-erase cell characteristic in a flash cell, in a flash memory device in which a source line is formed by a local interconnection method.
In order to accomplish the above object, a method of monitoring a source contact in a flash memory according to the present invention is characterized in that it comprises the steps of providing a flash memory device in which a source line is formed by a local interconnection method; connecting a common VG terminal to word lines, connecting a VD terminal to the first drain contact of drain contacts, making the remaining drain contacts floated, connecting a VSS terminal to the last common source contact of common source contacts connecting a VS terminal to the remaining source contacts; applying an over-erase voltage to each of the terminals so that all the cells become normally turned on; and applying a test voltage to each of the terminals so that current can flow from the VD terminal to the VSS terminal to confirm the current flow.